8 DISCOVERY REPORTS 



road that these larvae really belong to Sicyonella. In view of the fact that the genus 

 Petalidiinn is so close to Sergestes that it is scarcely separable (Burkenroad, 1937, p- 324) 

 it is most unlikely that the larvae would differ to any great extent, and we feel that 

 Mr Burkenroad's transference of these larvae to Sicyonella can probably be accepted. 



We have seen no larvae in the Discovery material which can be referred to either of 

 these genera. 



IDENTIFICATION OF THE LARVAE 



The three Elaphocaris stages differ so much from each other that it is impossible to 

 frame a key which will cover them all, and also our knowledge of them is not yet 

 sufficiently complete for such a key to be useful. The Mastigopus stages, again, change 

 so much with growth in the proportions of the eye, uropod, etc. that a key could only 

 be satisfactory if it could be made to apply to a single stage, and that is neither possible 

 nor desirable. On the other hand, the Acanthosoma stages differ but little from each 

 other and we have made a key to them which we hope may make identification easy. 



KEY TO THE ACANTHOSOMA STAGES 



1 . Abdominal somites without median ventral spines ... ... ... ■ • • ... 2 



Abdominal somites with two or more ventral spines; lateral spine of somite 5 very small 10 



2. Thorax without posterior dorsal spine ... ... ... ... ... ... ... 3 



Thorax with posterior dorsal spine ... ... ... ... ... ... ... ... 4 



3. Margin of carapace not denticulate ... ... ... ... . . . S . corniculum 



Margin of carapace denticulate S. crassus 



4. Exopod of uropod with setose part more than il times as long as bare part ... ... 5 



Setose part less than bare part or not more than i \ times as long ... ... ... 6 



5. Carapace with angular projection behind anterior lateral spine ... ... ... S. pectinatus 



Carapace without this projection ... ... ... ... ... ... ■ • • S. sargassi 



6. Eye long and slender, eyeball nearly round ... 7 



Eyeball large, asymmetrical ... ... ... ... ... ... ■■. ... ■■■ 8 



7. Lateral spines of somites 4 and 5 very long ... ... ... ... ... S.robustus 



These spines very short S. arcticus 



8. Lateral spines of thorax and abdomen very long, with long spinules; eyes short S. cormitiis 

 These spines short, with very small spinules; eyes very long ... ... ... ... 9 



9. Setose part of uropod shorter than bare part; endopod of uropod wider than exopod 



5. tenuiremis 

 Setose part longer than bare part ; endopod not wider than exopod ... ... S. atlanticus 



10. Ventral spines on somites i and 2 ... ... ... ... ... ... ... ... n 



Ventral spines on somites 1-3 ... ... ... ... ... S. armatus 



Ventral spines on somites 1-5 ... ... ... ■•• ■■■ S. edwardsi 



1 1 . Lateral spines of somites 4 and 5 very small ; eye nearly as long as scale ... S. vigilax 

 Lateral spine of somite 4 much larger than 5 ; eye about half as long as scale ... S. diapontius 



